anticodon

An anticodon is the three-base recognition tag on a transfer RNA — the “key” that fits the codon “lock” on the mRNA. When a codon and a matching anticodon meet at the ribosome, they pair up by the usual base-pairing rules, ensuring the correct amino acid is delivered.

Because the two strands of any base-paired sequence run in opposite directions, an anticodon reads complementary and antiparallel to its codon. For example, the codon AUG is recognized by a tRNA whose anticodon is CAU, lined up so that A pairs with U, U with A, and G with C.

Pairing at the third position is sometimes looser than strict rules predict — a phenomenon called wobble — so a single tRNA can read more than one codon for the same amino acid. This flexibility is part of why a cell needs fewer kinds of tRNA than there are codons.